Fluids used in the hydraulic fracturing of petroleum reservoirs and other applications of viscous fluids often require field analysis of the apparent viscosity of the fluid as a quality control check and as a parameter useful in designing a fracturing operation or the like. One type of viscometer used in determining the apparent viscosity of a fluid is manufactured by NL Industries, Inc. under the trademark "Fann." This type of viscometer utilizes a tubular rotor which rotates about a stationary cylindrical bob member which is connected to a torsion spring for deflection under the viscous drag of the fluid which is disposed in an annular space between the rotor and the bob. Deflection measurements taken at different rotational speeds of the rotor corresponding to different shear rates yields information which may be converted to shear stress so that the fluid flow behavior indexes may be calculated and the apparent viscosity determined.
However, field analysis of relatively viscous liquids of the type which have a higher viscosity with increasing temperature, such as used in hydraulic fracturing operations, are somewhat unreliable using the conventional rotary viscometer. Accordingly, there has been a need for an improved type of viscometer which provides for thorough mixing of the fluid being heated for analysis and which is particularly adapted for field use at a well site for analyzing certain types of fluid samples. The present invention satisfies this need and provides and improved apparatus and method for making field viscosity measurements of fluids, particularly of the type used in hydraulic earth formation fracturing operations.